1. Field of the Invention
Aspects of the present invention relate to an organic light emitting display device (OLED) and a method of fabricating the same, and more particularly, to an OLED and a method of fabricating the same, which can maximize transfer efficiency in formation of an organic layer by a laser induced thermal imaging (LITI) method and minimize pixel shrinkage caused by outgas sing.
2. Description of the Related Art
The OLED, among flat panel display devices, is a self-emissive display device emitting light by electrically exciting an organic compound. The OLED can be made lightweight and thin because it does not need a backlight used in an LCD, and can also be made at a low temperature by simple processes. Additionally, the OLED has a high response speed of 1 ms or less, low power consumption, a wide viewing angle, and high contrast.
The OLED includes an organic emission layer between an anode and a cathode, where a hole supplied by the anode and an electron supplied by the cathode combine in the organic emission layer to form an unstable hole-electron pair called an exciton which emits light by decaying.
OLEDs are classified into a bottom-emission type and a top-emission type depending on a direction from which light is emitted from the organic emission layer. If an OLED having a pixel driving circuit disposed therein is a bottom-emission type, an aperture ratio is significantly restricted by the large area of the substrate occupied by the pixel driving circuit. Thus, the top-emission type OLED has been introduced in order to enhance the aperture ratio.
FIG. 1 is a cross-sectional view of a conventional top-emission type OLED. Referring to FIG. 1, a buffer layer 110 is formed on a substrate 100 formed of glass or plastic. A thin film transistor including a semiconductor layer 120 having source and drain regions 120a and 120c and a channel region 120b, a gate insulating layer 130 and a gate electrode 140, is formed on the buffer layer 110.
An interlayer insulating layer 150 is formed on the entire surface of the substrate including the thin film transistor. Then, contact holes 155a and 155b are formed in the interlayer insulating layer 150 and the gate insulating layer 130 to expose portions of the source and drain regions 120a and 120c. 
Source and drain electrodes 160a and 160b are formed to be electrically connected to the source and drain regions 120a and 120c through the contact holes 155a and 155b. A planarization layer 170 is formed on the entire surface of the substrate including the source and drain electrodes 160a and 160b. A via hole 175 is formed in the planarization layer 170 to expose a portion of the drain electrode 160a. 
Next, a first electrode 180 contacting the drain electrode 160a through the via hole 175 is formed on the entire surface of the substrate. The first electrode 180 is a transparent electrode and includes an underlying reflective layer.
A pixel defining layer 190 is formed on the first electrode 180. The pixel defining layer is formed of an organic material, to a thickness of about 0.5 to 1 μm, and patterned to include an opening 200 exposing the first electrode 180. An organic layer (not illustrated) is formed on the first electrode. The organic layer includes at least an organic emission layer and may further include at least one of a hole injection layer, a hole transport layer, an electron transport layer and an electron injection layer. A second electrode (not illustrated) is formed on the entire surface of the substrate including the organic layer, and thus an OLED is completed.
The planarization layer removes a step of a pixel region in the top-emission OLED and is mainly formed of an organic material such as polyimide. Such a planarization layer can damage the organic layer due to outgassing, and thereby cause pixel shrinkage. Also, when the planarization layer formed of the organic material is combined with an encapsulated substrate, a bonding force decreases, thereby allowing air and moisture to penetrate from the outside, decreasing a device's lifespan.
Meanwhile, in the conventional OLED, the first electrode is formed to protrude from the surface of the planarization layer, thereby generating a step, and thus the following pixel defining layer should be thickly formed to surround an edge of the first electrode.
Here, due to the step between the thickly formed pixel defining layer and the first electrode, when the organic layer is formed by a laser induced thermal imaging (LITI) method, transfer efficiency decreases and deterioration of the organic layer is accelerated. In particular, since the organic layer is not transferred well at an edge part of the opening and tends to get cut off, an opening defect may occur.
However, when the pixel defining layer is formed of a thin organic layer which is about 2000 Å in thickness to improve the organic layer transfer efficiency, then when a transfer layer of a donor substrate is removed after forming the organic layer on the opening of the first electrode, the pixel defining layer may also be lifted off. This generates a short circuit between the first and second electrodes and causes unevenness and non-uniform dispersion when stacking the pixel defining layer.
Also, to exacerbate such a problem, when the pixel defining layer is an inorganic layer having a thickness of about 1000 Å, the inorganic material is incapable of fully filling a via hole. Alternatively, when the thickness of the inorganic layer is increased, a crack is generated at the edge of the first electrode and the periphery of the via hole owing to stress. Particularly, since the inorganic material has poor step coverage, the crack at the edge of the first electrode is caused by the step between the first electrode and the planarization layer, and thus a short circuit may be generated between the first and second electrodes.